Measuring and Modeling Sediment Production from Unpaved Roads, St. John,U.S.Virgin Islands

Previous studies have identified unpaved roads as the most important source of sediment on the island of St. John in the U.S. Virgin Islands, but were not able to evaluate the primary controlling factors or develop predictive models for planning and management. The goals of this study were to: (1) quantify the effect of different factors on sediment production from unpaved road surfaces; and (2) develop an empirical model for predicting sediment production from unpaved road segments. Sediment production rates from 22 road segments were intermittently monitored from July 1998 to May 2000, yielding a total of 83 measurements. Road segment slopes ranged from 1 to 21 percent, traffic varied from almost zero on abandoned roads to more than 250 vehicles per day, and there was considerable variability in the time since grading and the frequency of regrading. Five recording rain gauges provided rainfall amounts and erosivities for the different segments and measurement periods. Preliminary analyses show that sediment production is strongly related to both rainfall erosivity and the availability of fine particles on the road surface. Road surface erosion rates were related to the product of road drainage area and slope (Anderson and MacDonald, 1998), but this relationship is probably significant because area times slope is a surrogate for the erosive energy of road surface runoff. Road segments regraded within a year produced much more sediment than roads that had not been recently regraded, and this is probably due to the increased availability of fine particles. An empirical model was developed to predict sediment production from unpaved road segments as a function of total rainfall, road gradient or the area-slope product, and a discrete qualitative variable combining estimated road usage and time since grading. The application of this model to 22 road segments yielded road erosion rates ranging from 0.6 to 29 kg m-2 yr-1 (3.6 to 140 tons km-1 yr-1). These values are at the high end of the road erosion rates reported in the literature, but they are consistent with the high rainfall erosivities generated by hurricanes and tropical depressions. Limited data from undisturbed plots and other sources suggest that erosion rates for unpaved roads on St. John are three to four orders of magnitude higher than natural sediment production rates, and this sediment may pose a severe threat to the salt ponds, mangroves and marine resources that are of primary concern.